1. Field of the Invention
The present invention relates to a toner for visualizing a latent electrostatic image formed on an image bearing member by methods including electrophotography and electrostatic recording methods. In addition, the present invention also relates to a developer including the toner, an image forming method using the toner and a toner container containing the toner.
2. Discussion of the Background
U.S. Pat. No. 2,297,691 and published examined Japanese Patent Applications Nos. 49-23910 and 43-24748 disclose various kinds of methods for image developing by electrophotography. In general, electrostatic latent images, which are formed on an image bearing member using a method such as electrophotography, electrostatic recording, and electrostatic printing, are developed with a developer in order to be visualized.
For example, visual images are typically formed as follows:    (1) a latent electrostatic image is formed on an image bearing member such as photoreceptors which is made of photosensitive materials (latent image forming process);    (2) the latent electrostatic image is developed with a developer including a toner to form a toner image on the image bearing member (developing process);    (3) the toner image is transferred onto a receiving material, such as paper, optionally via an intermediate transfer medium (transfer process); and    (4) the toner image on the receiving material is fixed upon application of heat, pressure, solvent vapor, etc. to form a hard copy (fixing process).
As methods for developing latent electrostatic images, there is a wet developing method, which uses a liquid developer prepared by finely dispersing various kinds of dyes and pigments in an insulative organic solution. Another typical method for developing latent electrostatic images is a dry developing method, such as a cascade method, a magnetic brush method, and a powder cloud method, which uses a dry developer (hereinafter referred to as toner) made by dispersing colorants such as carbon black in a natural or synthetic polymer. Recently the dry developing methods have been widely accepted. The fixing processes used in the dry developing methods are typically a contact heating fixing process, such as a heat roll fixing process, or a belt fixing process, since these processes are energy efficient. However, for the heat roll fixing process, there is a problem in that toners tend to excessively fuse and attach to the heat roll when the temperature thereof is too high. To the contrary, when the temperature of the heat roll is too low, toners tend not to fuse well, resulting in insufficiency of fixing. Therefore, toners having properties of a high hot offset temperature (hot offset resistance) and a low temperature fixing are required in light of the lowered energy consumption and miniaturization of developing devices.
In order to satisfy the hot offset resistance, ultra high molecular weight resins, which are sufficiently tough and elastic, are required. In contrast, low energy and sufficient fusibility and viscosity are necessary to have the property of low temperature fixing. Therefore low molecular weight resins having a low glass-transition temperature are required. Various approaches have been devised to obtain materials and methods satisfying both properties. Japanese Patent No. 2986820 and published unexamined Japanese Patent Application No. 2000-194160 disclose methods of using a binder resin including both a low molecular weight component and high molecular weight component (e.g., a cross-linked and branched ultra-high molecular weight component). However, these methods have drawbacks in that (1) the added amount of the ultra-high molecular weight component is limited due to its poor solubility or mutual solubility of the high molecular weight component, and the low molecular weight component and (2) the ultra-high molecular weight component, which has a sufficient elasticity, worsens the property of low temperature fixing.
Therefore, the need still exists for a toner having both properties of hot offset resistance and low temperature fixing.